As polyesterether copolymer is excellent in processability and properties, it collects attention in engineering plastic fields such as automobile and electronic electric parts industries. As methods of producing these polymers, there have been known the following methods: a method for carrying out polycondensation after transesterification, with the polyether being allowed to coexist during transesterification, between an aromatic dicarboxylic acid ester and a glycol; as shown in Laid-open Publication No. 198527/83, a method for carrying out polycondensation after transferring into a polycondensation vessel charged beforehand with a polyether, esterified products from a direct esterification between an aromatic dicarboxylic acid and a glycol; or, as shown in Laid-open Publication No. 19696/73, a method for carrying out polycondensation by adding a polyether after oligoesterification was made by adding alkylene glycol to polyester. These methods, however, contain the following problems: requirement of complicated processes and facilities, no avoidance of poor quality and operability decrease caused by the generation of bu-products of the polyether, such as diethylene glycol, due to longer reaction time, and difficulty in obtaining high polymerization degree due to higher decomposing rate during the reaction in a system containing a large quantity of thermally unstable polyether.
In light of the foregoing situation, the present inventors have made an extensive series of studies to eliminate the above problems to thus provide the present invention.